Anxiety disorders are the most common form of psychiatric illness and exact a huge toll on America's health. Current treatments are often inadequate suggesting more effective, more specific therapies are needed. Clinical studies have firmly established that CO2 inhalation triggers anxiety and panic attacks, and that patients with anxiety disorders are hyper-responsive to CO2. These findings suggest that a better understanding of the molecular mechanisms underlying CO2 sensitivity could lead to novel insight into the causes of anxiety disorders and possibly lead to better treatments. Because CO2 sensitivity has been explored primarily in clinical studies, which are restricted in their ability to identify molecular mechanisms, there is a significant need for animal models to probe the mechanisms underlying CO2 sensitivity. In this proposal we address this need for animal models of CO2-evoked fear, by modeling CO2 behavioral and physiological responses in mice. We investigate the hypothesis that CO2 inhalation lowers brain pH, which activates pH-sensitive receptors in the fear circuit, which in turn increase the behavioral and physiological manifestations of fear, anxiety, and panic. This project may be critical for helping to explain the long recognized, but poorly understood clinical phenomenon of CO2 sensitivity. In addition, these studies are likely to have broader implications. Our preliminary data suggest that CO2 activates novel signaling pathways underlying anxiety disorders, and that these pathways might be therapeutically targeted to prevent anxiety disorders and reduce their symptoms.